Acute coronary syndromes are among the major causes of mortality and morbidity in the world. Millions of patients are treated annually with chest pain of acute onset. In this situation it is extremely important to quickly identify when this pain is due to myocardial ischemia so that adequate care may be started. Myocardial ischemia is a clinical condition characterized by reduced blood supply to the heart. Currently, there are no effective diagnostic methods for this disease. Presently, the diagnosis is based on common non-invasive tests such as exercise testing, myocardial perfusion scintigraphy, however, these tests are expensive and have limited sensitivity and specificity (Parikh; Lemos, 2006). Also, a large part myocardial ischemia patients go undiagnosed due to the absence of accurate diagnostic tests for myocardial ischemia without necrosis of the heart muscle (Pope at al, 2000; McCaig; Burt, 2002; Lloyd-Jones et al, 2009).
In the United States, the FDA has approved the use of ischemia modified albumin (IMA) as a marker for ischemia. This marker consists of the loss of affinity of albumin for cobalt after myocardial ischemia. However, this marker is not sufficiently specific, as it has been shown to be elevated in several clinical situations such as in renal failure, stroke and heavy-load aerobic exercise (Debashis I, et al, 2004; Lippi, et al, 2005; Morrow, et al, 2007; Singh et al, 2010).
Recently, the use of the levels of cytoplasmic cardiac troponins in serum as markers for myocardial injury has been suggested, Cardiac troponins are specific to cardiac myocytes, however, due to the high sensitivity of the assay, these proteins are also detectable in the serum of healthy patients and in other heart diseases as well as acute coronary syndromes or clinical situations where myocardial ischemia occurs (Eggers, et al, 2009; Reichlin et al, 2009; Masson et al, 2010; Hochholzer et al, 2011).
Thus, the currently available cardiac ischemia markers are either nonspecific because they are present in other organs and tissues other than the myocardium, and/or they are elevatedin other clinical situations other than myocardial ischemia.
Due to the prominent lack of markers of cardiac ischemia, the diagnosis and early treatment may occur at a very late stage in patients with unstable angina. On the other hand, in patients with non-ischemic chest pain, the uncertainty about the diagnosis may require hospital admission for investigation which results in undesirable excessive health care costs.
In addition to emergency testing, an accurate marker of myocardial ischemia is also useful in ambulatory management of patients in the diagnosis of stable angina, especially in association with exercise testing or stress myocardial scintigraphy.
Interestingly, cardiovascular diseases, especially acute coronary syndromes, are strongly associated with an oxidative imbalance, leading to increased free radicals and increased reactive oxygen and nitrogen species. Thus, in such diseases, there is an increase in the production of nitric oxide, superoxide and a change in the antioxidant pathways, leading to nitroxidative and oxidative stress (Levrand et al, 2006; Peluffo, Radi, 2007; Aslan, Dogan, 2011).
As a result of the increase of these reactive species, intermediate species such as peroxynitrite and formed. Peroxynitrite, among other actions, leads to the formation of nitrated proteins by the addition of a NO2 group to the phenolic ring of the tyrosine residues (Ischiropoulos, 2009).
This nitration may alter the protein in several ways: the protein may become resistant to degradation, it may activate the immune system, it may become inactivated or it may acquire a new function. These changes in protein activity can impact the pathophysiology of the cardiovascular system (Abello et al, 2009).
Several proteins have been identified as being nitrated in different compartments of the cardiovascular system, such as fibrinogen, plasmin and Apo-1 in the plasma; Apo-B, cyclooxygenase, prostaglandin synthase and Mn-SOD in the vessel walls; myofibrillar creatine kinase and α-actinin in the myocardium. These nitrated proteins were associated to cardiovascular disease (Aslan, Dogan, 2011).
In view of this, nitrated proteins are emerging as new markers for cardiovascular disease. However, protein nitration also occurs in other pathophysiological situations involving increased reactive oxygen and nitrogen species, such as in diabetes, stroke, neurodegenerative diseases, kidney disease and other diseases associated with ischemia/reperfusion and inflammation (Gole et al, 2000; Castegna et al, 2003; Turko et al, 2003, Ahmed et al, 2005; Heffron et al, 2009; Isobe et al, 2009; Choi et al, 2010; Tyther et al, 2011; Piroddi et al, 2011). Thus, in the search for a nitrated biomarker that is specific to cardiovascular disease it is important that the modified protein target is expressed only or mainly in cardiac tissue.
Troponins are cytoplasmic proteins that have specific cardiac muscle isoforms and are found in the circulation in very low concentrations in healthy subjects and that have its concentration increased in situations of stress to the heart muscle, such as necrosis of the cardiomyocytes. Cardiac troponin I is a cytoplasmic protein unique to cardiac myocytes and is one of three subunits that form the troponin protein complex. This subunit is responsible for sensitivity to calcium and regulates muscle contraction. Troponin I binds to actin and inhibits the ATPase activity of actomiosin in the absence of calcium (Bhaysar et al, 1996).